1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One such fixing device includes a multi-roller, belt-based fuser assembly that employs an endless fuser belt entrained around multiple rollers, paired with a pressure roller pressed against the outer surface of the fuser belt to form a fixing nip therebetween. The fuser belt is held on a heat roller equipped with an internal heater, which heats the length of the fuser belt through contact with the heat roller. At the fixing nip, a toner image on an incoming recording sheet is fixed in place with heat from the fuser belt and pressure from the pressure roller.
Another type of fixing device includes a film-based fuser assembly that employs a fuser belt formed of thin heat-resistant film cylindrically looped around a stationary, ceramic heater, which is paired with a pressure roller that rotates while pressing against the stationary heater through the fuser belt to form a fixing nip therebetween. At the fixing nip, the pressure roller rotates to advance the fuser belt together with an incoming recording sheet, while the stationary heater heats the recording sheet via the fuser belt, so that a toner image is fixed in place with heat from the stationary heater and pressure from the pressure roller.
Of the two types of fuser assembly described above, the film-based assembly is superior to its counterpart in terms of processing speed and thermal efficiency. Owing to the heat-resistant film which exhibits a relatively low heat capacity and therefore can be swiftly heated, the film-based fuser assembly eliminates the need for keeping the heater in a sufficiently heated state when idle, resulting in a shorter warm-up time and smaller amounts of energy wasted during standby, as well as a relatively compact size of the fuser assembly.
By contrast, the multi-roller belt fuser, although advantaged over a conventional roller-based fuser, involves a substantial warm-up time to heat the fixing nip to a temperature sufficient for fusing toner and first-print time to complete an initial print job upon activation, limiting its application to relatively slow imaging systems.
Overcoming the limitation of the belt-based fixing device, the film-based fixing device finds applications in high-speed, on-demand compact printers that can promptly execute a print job upon startup with significantly low energy consumption.
Although generally successful for its intended purpose, the fixing device using a thin film fuser also has drawbacks. One drawback is its vulnerability to wear, where the heat-resistant film has its inner surface repeatedly brought into frictional contact with the surface of the stationary ceramic heater. The frictionally contacting surfaces of the film and the heater readily chafe and abrade each other, which, after a long period of operation, results in increased frictional resistance at the heater/film interface, leading to disturbed rotation of the fuser belt, or increased torque required to drive the pressure roller. If not corrected, such defects can eventually cause failures, such as displacement of a printed image caused by a recording sheet slipping through the fixing nip, and damage to a gear train driving the fixing members due to increased stress during rotation.
Another drawback is the difficulty in maintaining a uniform processing temperature throughout the fixing nip. The problem arises where the fuser film, which is once locally heated at the fixing nip by the heater, gradually loses heat as it travels downstream from the fixing nip, so as to cause a discrepancy in temperature between immediately downstream from the fixing nip (where the fuser belt is hottest) and immediately upstream from the fixing nip (where the fuser belt is coldest). Such thermal instability adversely affects fusing performance of the fixing device, particularly in high-speed applications where the rotational fixing member tends to dissipate higher amounts of heat during rotation at a high processing speed.
The former drawback of the fixing device has been addressed by another conventional fixing device, which uses a lubricant, such as a low-friction sheet of fiberglass impregnated with polytetrafluoroethylene (PTFE), disposed between the contacting surfaces of a stationary pressure pad and a rotatable fixing belt. In this fixing device, the rotatable fixing belt is looped for rotation around the stationary pressure pad, while held in contact with an internally heated, rotatable fuser roller that has an elastically deformable outer surface. The pressure pad is spring-loaded to press against the fuser roller through the fixing belt, which establishes a relatively large fixing nip therebetween as the fuser roller elastically deforms under pressure.
According to this arrangement, provision of the lubricant sheet prevents abrasion and chafing at the interface of the stationary and rotatable fixing members, as well as concomitant defects and failures of the fixing device. Moreover, the relatively large fixing nip translates into increased efficiency in heating a recording sheet by conduction from the fuser roller, which allows for designing a compact fixing device with reduced energy consumption.
However, the conventional method does not address the thermal instability caused by locally heating the fixing belt at the fixing nip, as is the case with the conventional fixing device. Further, this method involves a fixing roller that exhibits a relatively high heat capacity and therefore takes time to heat up to a desired processing temperature, leading to a longer warm-up time. Hence, although designed to provide an increased thermal efficiency through use of an elastically deformable fuser roller, the conventional method fail to provide satisfactory fixing performance for high-speed, on-demand applications.
To cope with the problems of the fixing device using a cylindrically looped, rotatable fixing belt, several methods have been proposed.
For example, one conventional method proposes a fuser assembly that employs a stationary tubular belt holder of thermally conductive material around which a fuser belt is retained in its generally cylindrical shape. The belt holder is equipped with a resistive heater such as a ceramic heater disposed inside the tube so as to heat the entire length of fuser belt rotating around its circumference.
According to this method, the thermal belt holder, which is formed by bending a thin sheet of metal into a tubular configuration, can swiftly conduct heat to the fuser belt, while guiding substantially the entire length of the belt along the outer circumference thereof. Compared to a stationary heater or heated roller that locally heats the fuser belt or film solely at the fixing nip, using the thin-walled conductive belt holder allows for heating the fuser belt swiftly and uniformly, resulting in shorter warm-up times which meet high-speed, on-demand applications.
One drawback encountered when using a tubular belt holder to heat a fuser belt is the difficulty in maintaining uniform spacing between the fuser belt and the belt holder. That is, the elastic fuser belt during rotation occasionally moves too far from the surface of the belt holder to conduct appropriate amounts of heat from the belt holder to the fuser belt. The lack of conduction can cause the metal-based belt holder to locally overheat and burn, resulting in an increased torque of the fuser belt rotating along the damaged surface.
Another conventional method employs a cylindrically looped fuser belt paired with a pressure roller pressed against the fuser belt to form a fixing nip, as well as a stationary, resistive heater in the form of a thin-walled pipe of metal that exhibits a certain resistivity to generate heat when electrified. The resistive heater is installed within the loop of fuser belt with a small spacing in a radial direction, so that their adjoining surfaces do not press against each other, and radiates heat over the entire length of the fuser belt rotating around the metal pipe.
According to this method, holding the fuser belt in close proximity with the resistive heater allows for good imaging performance at high processing speeds, which results in shorter warm-up time and first-print time of the belt-based fixing device. Moreover, keeping the fuser belt and the resistive heater slightly apart prevents abrasion and other concomitant failure of the fuser belt and the resistive heater in high-speed applications.
One drawback of this method is that the flexible fuser belt suffers from variations in temperature as it occasionally contacts or moves away from the surface of the resistive heater, as is the case with the one entrained around the tubular belt holder described above. In particular, the fuser belt can locally overheat where the fuser belt, disposed in close proximity with the resistive heater for obtaining high thermal efficiency, contacts the heater to conduct excessive amounts of heat therebetween. Such localized heating of the fuser belt results in poor imaging performance due to non-uniform distribution of heat across the fixing nip.
Another drawback is that the resistive heater can wear and break as it undergoes repeated flexion or stress caused by rotational vibration transmitted from the pressure roller through the fuser belt. Once broken, the metal-based resistive heater no longer gives off sufficient heat to the fuser belt, resulting in defective fusing performance of the fixing device.